Re: plaster of paris mold for aluminum


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Posted by bpfink on August 30, 1999 at 21:17:15:

In Reply to: plaster of paris mold for aluminum posted by Rob Frink on August 30, 1999 at 08:06:22:

You WILL need to mix in some sand with the plaster (and plaster of paris is fine) as it by itself can not withstand the shock of burning out without the sand aggregate.. even though that is a fine aggregate.

Use about a 50 / 50 mix and silica is fine.

This is highly oversimplified so would suggest you read a handful of these posts as well or even check out some book.

YOU DO have to burn out the plaster investment even if it is an opened mold and you have nothing in it to burn out... you still do and it's called water.

When you mix plaster with water it reacts both mechanically and chemically. The mechanical H2O is the greater of the two and

can in time or dryness or sun remove itself nearly all the way. The chemically combined H2O however is staying there and you need to do a slow bake / burnout or the hot metal will convert and release it as steam and be a major burp / explosion / or at least a fully ruined casting that gases to absurd pitted proportions.

In green sand that H2O can escape right through the sand itself... but not in a plaster investment.

I would recommend at least a 600 degree F burnout and to hold it there long enough to be sure. Not knowing what sizes you are referring to??? from 6hrs. to several days. I do investments and the my smaller ones are about 200 lb per mold and these take 3 days and nites and I go to 900 F on the last day. (Hey, some prefer up to 1500F but I abandoned that temp many years ago).

A larger mold to 2000 lb will have a burnout time of 5 to 7 days (that is a 24 hr day x 7) but these things all depend on what is being burned out also.

Good luck and be careful. Accidents with these materials could be real disasters.


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